patch 8.1.1210: support for user commands is spread out

Problem:    Support for user commands is spread out. No good reason to make
            user commands optional.
Solution:   Move user command support to usercmd.c.  Always enable the
            user_commands feature.
diff --git a/src/Make_morph.mak b/src/Make_morph.mak
index 65cf844..a5ce62b 100644
--- a/src/Make_morph.mak
+++ b/src/Make_morph.mak
@@ -81,6 +81,7 @@
 	term.c							\
 	ui.c							\
 	undo.c							\
+	usercmd.c						\
 	userfunc.c						\
 	version.c						\
 	window.c						\